Add adaptive FPS and honest device reachability during streaming

DDP uses fire-and-forget UDP, so when a WiFi device becomes overwhelmed
by sustained traffic, sends appear successful while the device is
actually unreachable. This adds:

- HTTP liveness probe (GET /json/info, 2s timeout) every 10s during
  streaming, exposed as device_streaming_reachable in target state
- Adaptive FPS (opt-in): exponential backoff when device is unreachable,
  gradual recovery when it stabilizes — finds sustainable send rate
- Honest health checks: removed the lie that forced device_online=true
  during streaming; now runs actual health checks regardless
- Target editor toggle, FPS display shows effective rate when throttled,
  health dot reflects streaming reachability, red highlight when
  unreachable
- Auto-backup scheduling support in settings modal

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/wled_controller/static/locales/zh.json

@@ -169,6 +169,7 @@
   "device.metrics.device_fps": "设备刷新率",
   "device.health.online": "在线",
   "device.health.offline": "离线",
+  "device.health.streaming_unreachable": "流传输期间不可达",
   "device.health.checking": "检测中...",
   "device.tutorial.start": "开始教程",
   "device.tip.metadata": "设备信息（LED 数量、类型、颜色通道）从设备自动检测",
@@ -912,6 +913,8 @@
   "targets.brightness_vs.none": "无（设备亮度）",
   "targets.min_brightness_threshold": "最低亮度阈值：",
   "targets.min_brightness_threshold.hint": "当有效输出亮度（像素亮度 × 设备/源亮度）低于此值时，LED完全关闭（0 = 禁用）",
+  "targets.adaptive_fps": "自适应FPS：",
+  "targets.adaptive_fps.hint": "当设备无响应时自动降低发送速率，稳定后逐步恢复。推荐用于信号较弱的WiFi设备。",
 
   "search.open": "搜索 (Ctrl+K)",
   "search.placeholder": "搜索实体... (Ctrl+K)",
@@ -943,5 +946,25 @@
   "settings.restore.error": "恢复失败",
   "settings.restore.restarting": "服务器正在重启...",
   "settings.restore.restart_timeout": "服务器未响应。请手动刷新页面。",
-  "settings.button.close": "关闭"
+  "settings.button.close": "关闭",
+
+  "settings.auto_backup.label": "自动备份",
+  "settings.auto_backup.hint": "自动定期创建所有配置的备份。当达到最大数量时，旧备份会被自动清理。",
+  "settings.auto_backup.enable": "启用自动备份",
+  "settings.auto_backup.interval_label": "间隔",
+  "settings.auto_backup.max_label": "最大备份数",
+  "settings.auto_backup.save": "保存设置",
+  "settings.auto_backup.saved": "自动备份设置已保存",
+  "settings.auto_backup.save_error": "保存自动备份设置失败",
+  "settings.auto_backup.last_backup": "上次备份",
+  "settings.auto_backup.never": "从未",
+
+  "settings.saved_backups.label": "已保存的备份",
+  "settings.saved_backups.hint": "存储在服务器上的自动备份文件。下载到本地保存，或删除以释放空间。",
+  "settings.saved_backups.empty": "没有已保存的备份",
+  "settings.saved_backups.restore": "恢复",
+  "settings.saved_backups.download": "下载",
+  "settings.saved_backups.delete": "删除",
+  "settings.saved_backups.delete_confirm": "删除此备份文件？",
+  "settings.saved_backups.delete_error": "删除备份失败"
 }